In 1996 T. G. Zimmerman reported work on a Personal Area Network for enabling near-field intrabody commnication. IBM Systems Journal, Vol. 35, No. 3&4, 1996-MIT Media Lab, 0018-8670/96.COPYRGT. IBM, Personal Area Networks: Near-field Intrabody Communication. That article is hereinafter excerpted and is incorporated by reference herein in its entirety. The initially conceived function of the network was to enable body carried electronic devices, such as cellular phones, personal digital assistants (PDAs), pocket video games, and pagers, to share data. The concept of Personal Area Networks (PANs) was stated to demonstrate how electronic devices on and near the human body can exchange digital information by capacitively coupling picoamp currents through the body. A low-frequency carrier (less than 1 megahertz) is used so no energy is propagated, minimizing remote eavesdropping and interference by neighboring PANs. A prototype PAN system allows users to exchange electronic business cards by shaking hands. Using a small transmitter approximately the size of a deck of cards embedded with a microchip, and a slightly larger receiving device, it is possible to transmit a pre-programmed electronic business card between two people via a simple handshake.
The natural salinity of the human body makes it an excellent conductor of electrical current. PAN technology takes advantage of this conductivity by creating an external electric field that passes a very small current through the body, over which data is carried. The current used is one-billionth of an ampere (one nanoamp), which is lower than the natural currents already in the body. The speed at which the data is transmitted is equivalent to a 2400 baud modem. Theoretically 400,000 bits per second could be communicated using this method.
The original researchers reportedly envisioned PAN technology as initially being applied in three ways:
To pass simple data between electronic devices carried by two human beings, such as an electronic business card exchanged during a handshake.
To exchange information between personal information and communications devices carried by an individual, including cellular phones, pagers, personal digital assistants (PDAs) smart cards. For example, upon receiving a page, the number could be automatically uploaded to the cellular phone, requiring the user to simply hit the "send" button.
To automate and secure consumer business transactions. Among the examples proffered by the developers:
A public phone equipped with PAN sensors would automatically identify the user, who would no longer have to input calling card numbers and PINs.
By placing RF (radio frequency) sensors on products, such as rental videos, stores could essentially eliminate counter lines and expedite rentals and sales. The customer would simply carry the selected videos through a detecting device that would automatically and accurately identify the customer and his selections, and then bill his account accordingly.
Health service workers could more safely and quickly identify patients, their medical histories and unique medicinal needs by simply touching them. This application would be particularly helpful in accident situations or where the patient is unable to speak or communicate.
Near-field PAN devices can operate at very low frequencies (0.1 to 1 megahertz) that can be generated directly from inexpensive microcontrollers. For example, the prototype PAN transmitter operated at 330 kilohertz (KHz) at 30 volts with a 10-picofarad electrode capacitance, consuming 1.5 milliwatts discharging the electrode capacitance. A majority of the energy is conserved or recycled using a resonant inductance-capacitance (LC) tank circuit. The PAN is based on the seven-layer ISO 7498 network standard and concerns the physical, second, and third layers.
FIG. 1 shows a PAN transmitter communicating with a PAN receiver. Both devices are battery powered, electrically isolated, and have a pair of electrodes. The PAN transmitter capacitively couples a modulating picoamp displacement current through the human body to the receiver. The return path is provided by the earth ground, which includes all conductors and dielectrics in the environment that are in close proximity to the PAN devices. The earth ground needs to be electrically isolated from the body to prevent shorting of the communication circuit.
In FIG. 2 the PAN transmitter is modeled as an oscillator, and the receiver is modeled as a differential amplifier. The basic principle of a PAN communication channel is to break the impedance symmetry between the transmitter electrodes and receiver electrodes. The transmitter's and receiver's intraelectrode impedances are ignored since the former is a load on an ideal voltage source and the latter is modeled as an open circuit. The four remaining impedances are labeled A, B, C, and D.
The circuit is rearranged to show how PAN device communication works by breaking the symmetry between the four electrodes. The circuit is a Wheatstone bridge where any imbalance of the relationship A/B=C/D will cause a potential across the receiver. Since the ratios must be exactly equal in order to null the circuit, and body-based PAN devices are constantly in motion, there will nearly always be an electrical communication path, as long as the receiver is sensitive enough to detect the imbalance.
A more detailed electrical model is derived by identifying all the electric field paths in the system. Electric fields exist between bodies at different potentials. FIG. 3 illustrates an electric field model of a PAN transmitter T communicating with a PAN receiver R. A small portion of the electric field G reaches the receiver R.
The transmitter T electrode closest to the body tb has a lower impedance to the body than the electrode facing toward the environment te. This enables the transmitter T to impose an oscillating potential on the body, relative to the earth ground, causing electric fields A, B, C, D, and E.
Similarly the impedance asymmetry of the receiver electrodes (rb and re) to the body and environment allow the displacement current from electric fields F and G to be detected. Since the impedance between the receiver electrodes is nonzero, a small electric field H exists between them.
The electric fields model is used to produce the electric circuit shown in FIG. 4. Some typical component values are shown for watch-based PAN devices. Referring to FIG. 3, the transmitter T capacitively couples to receiver R through the body (modeled as a perfect conductor). The earth ground provides the return signal. The circuit reveals that body capacitance to the environment E degrades PAN communication by grounding the potential that the transmitter T is trying to impose on the body.
The circuit model also suggests that feet are the best location for PAN devices, providing large electrodes in close proximity to the body and environment, respectively. This is particularly true for the environment electrode (te or re), which is the weakest link (largest impedance) in the circuit. The location also suggests a novel power source, i.e. PAN devices embedded in shoe inserts that extract power from walking. An adult dissipates several hundred milliwatts while walking. A piezoceramic pile charging a capacitor at an efficiency as low as 10 percent can provide enough power for a PAN device.
PAN devices can take the shape of commonly worn objects, such as watches, credit cards, eyeglasses, identification badges, belts, waist packs, and shoe inserts. Head-mounted PAN devices can include headphones, hearing aids, microphones, and head-mounted displays. Shirt pocket PAN devices may serve as identification badges. The wristwatch is a natural location for a display, microphone, camera, and speaker. A waist pouch can carry a PDA, cellular phone, keypad, or other devices that are large and heavy. PAN devices incorporating sensors can provide medical monitoring for such bodily functions as heartbeat, blood pressure, and respiratory rate. Pants pockets are a natural location for wallet-based PAN devices to store information and identify the possessor. Shoe inserts can be self-powered and provide a data link to remote PAN devices located in the environment, such as workstations and floor transponders that detect the location and identity of people.
The -3 dB bandwidth of the prototype PAN receiver is 400 KHz (100 KHz to 500 KHz), resulting in a maximum channel capacity of 417 kilobits (Kbits) per second, assuming a robust signal-to-noise ratio of 10. The PAN transceiver prototype implements a 2400 bits-per-second modem.
A PAN prototype was developed to demonstrate the digital exchange of data through a human body using battery-powered low-cost electronic circuitry. The detector is a current amp (gain=106) followed by an analog bipolar chopper controlled by a digital microcontroller, as shown in FIG. 4. The detector synchronously integrates the tiny received displacement current (e.g., 50 picoamperes, 330 KHz) into a voltage that can be measured by a slow, low-resolution analog-to-digital converter (50 KHz, 8 bits) provided by the microcontroller. The PAN transceiver uses five off-the-shelf components. The analog components and microcontroller can be combined into a single CMOS (complementary metal-oxide semiconductor) integrated circuit to produce a low-cost integrated PAN transceiver.
The transmitter is an LC tank (Q=6) made from a surface-mount inductor and the inherent electrode capacitance. The resonant tank circuit produces a clean sine wave output from a square wave input, minimizing RF harmonics, and boosts the output voltage in proportion to the Q of the tank. The transmit voltage can also be digitally programmed by varying the pulse width of the driving square wave. The integrator is discharged after every message bit (integrate-and-dump filtering) to minimize intersymbol interference.
Two modulation strategies were examined for PAN communication: on-off keying and direct sequence spread spectrum. On-off keying turns the carrier on to represent a message bit one and turns the carrier off for a message bit zero. The signal-to-noise performance is improved by increasing the transmit voltage. Direct sequence spread spectrum modulates the carrier with a pseudonoise (PN) sequence, producing a broadband transmission much greater than the message bandwidth. Symbol-synchronous PN modulation is used where a message bit one is represented by transmitting the entire PN sequence, and a message bit zero is represented by transmitting the inverted PN sequence. The signal-to-noise performance increases with the length of the PN sequence.
The prototype hardware was capable of detecting either on-off keying or direct sequence spread spectrum, determined by microcontroller coding. For on-off keying the bipolar chopper switches are driven at the carrier frequency, and the integrated result is compared to a fixed threshold to determine the value of the message bit. Quadrature detection is implemented by performing two sequential integrations, at 0 and 90 degrees phase, for each message bit. For spread spectrum the switches are driven by the PN sequence, and the integrated result, which is the correlation, is compared to two thresholds. If the correlation is greater than a positive threshold, the message bit is one. If the correlation is less than a negative threshold, the message bit is zero. If the correlation is between these thresholds (the dead zone), no message bit is received.
Once the message has been successfully received and demodulated, the microcontroller transmits the message to a host computer over an optical link (not shown), which electrically isolates the transceiver allowing evaluation and debugging independent of an electrical ground reference.
The demonstration prototype of the PAN system consisted of a battery-powered transmitter and receiver, and a host computer running a terminal program. The PAN prototypes measure 8.times.5.times.1 centimeters, about the size of a thick credit card. The transmitter contains a microcontroller that continuously transmits stored ASCII characters representing an electronic business card. The devices are located near the feet, simulating PAN shoe inserts. When the persons communicating are in close proximity, particularly when they shake hands, an electric circuit is completed, allowing picoamp signals to pass from the transmitter through one body, to the other body, to the receiver, and back through the earth ground. ASCII characters are sent to the receiver, demodulated, and sent via serial link to the host computer where they are displayed. Thus, when the subjects shake hands, one downloads his or her electronic business card to the other.
This PAN development provides a personal area network (PAN) device and system which is capable of transferring data from body to body through touch contact or mere close proximity. Available publications regarding the development suggest a number of areas of potential application. Among these is the automation and securing of consumer business transactions. One suggestion is a public phone equipped with PAN sensors which could automatically identify the user, who would no longer have to input calling card numbers and PINs. It is stated that this application would significantly reduce fraud and make calling easier and more convenient for users.
While this mode of using the personal area network or PAN devices would provide an improvement in the convenience of using public phones and would contribute to fraud reduction, it does not address the serious problem which is encountered in the case of a lost or stolen credit card in the possession of an unauthorized person. The suggested PAN technique would verify the card (PAN device) and any PIN contained within the PAN device. However this PAN technique would do nothing to cope with the problem posed by an individual in wrongful possession of the PAN and its data contents. In addition, the proposed application of the personal area network development to telecommunications does not extend beyond the limited identification function expressed.